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1. Field of the Invention
This invention relates generally to precisely positioning heavy objects, and more particularly to precisely positioning a test head of an electronic automatic test system for docking the test head with a prober, handler, or other peripheral for testing electronic devices.
2. Description of Related Art
Manufacturers of semiconductor chips and assemblies use automatic test equipment (xe2x80x9cATExe2x80x9d) to verify the performance of devices before the devices are shipped to customers. ATE systems typically include a xe2x80x9ctest headxe2x80x9d and a xe2x80x9ctester body.xe2x80x9d The test head houses portions of the test system that are preferably located as close as possible to the device under test, and connects to the tester body via one or more cables. For testing electronic devices, the test head connects or xe2x80x9cdocksxe2x80x9d with a peripheral. The peripheral feeds a series of devices to the ATE system for testing, and the ATE system tests the devices.
Constraints affecting semiconductor test processes often make it impractical to move the peripheral to the test head. In most manufacturing facilities, therefore, the peripheral that feeds the chips remains stationary, and the test head is moved into position for docking with the peripheral.
A device called a xe2x80x9cmanipulatorxe2x80x9d is used to move the test head to the peripheral. As is known, a common type of manipulator is the fork-arm manipulator, an example of which is shown in FIG. 1. Here, a manipulator 100 holds a test head 112 from its sides via fork arms 114. The manipulator 100 raises and lowers the test head 112 on a linear stage 118, and rotates the test head about a twist axis upon a twist gear 120. A fork arm manipulator like the one shown in FIG. 1 is disclosed in U.S. Pat. No. 5,949,002, entitled, xe2x80x9cManipulator for Automatic Test Equipment with Active Compliance,xe2x80x9d which is assigned to Teradyne, Inc., of Boston, Mass., and is hereby incorporated by reference.
FIG. 2 shows another type of manipulator used for positioning a test head. Rather than using fork arms, the manipulator 200 of FIG. 2 supports a test head 210 internally. The test head 210 is mounted to a central stiffener 212, which is, in turn, mechanically coupled to the manipulator via a central blade (not shown), which extends approximately through the center of the test head. The manipulator 200 can raise and lower the test head 210 on linear bearings 224, and can rotate the test head in the twist direction via twist bearing 214. It can also swing the test head 210 via a swing bearing 222. A manipulator like the one shown in FIG. 2 is disclosed in U.S. patent application Ser. No. 09/615,292, entitled xe2x80x9cAUTOMATIC TEST MANIPULATOR WITH SUPPORT INTERNAL TO TEST HEAD.xe2x80x9d This application is also assigned to Teradyne, Inc. and is hereby incorporated by reference.
Both types of manipulators generally include actuators such as motors (not shown) on their respective bearings and linear stages. The actuators move the test head to the peripheral, and orient the test head for docking. The test head is then docked with the peripheral by finely adjusting the position and orientation of the test head.
Manipulators commonly provide a range of xe2x80x9ccompliancexe2x80x9d that allows a test head to be rotated about one or more axes as the test head and peripheral are being docked. The range is xe2x80x9ccompliantxe2x80x9d because the test head literally complies with forces applied to the test head, which during docking tend to cause the mating surface of the test head to become coplanar with the mating surface of the peripheral.
In the fork arm manipulator of FIG. 1, the test head 112 can be made to nod compliantly in a xe2x80x9ctumblexe2x80x9d direction (NU and ND) by rotating the test head about pins 116. One pin 116 is provided within each fork arm on each side of the test head 112. The test head can also be made to turn compliantly in a xe2x80x9cthetaxe2x80x9d direction ("THgr"L and "THgr"R), via the movement of mechanisms within each fork arm, which allow the pins 116 to move slightly back and forth along the length of each fork arm. Opposing movements of the pins 116 on opposite fork arms effects compliant theta rotation.
The test head 210 of FIG. 2 can compliantly rotate in theta, tumble, and twist via a spherical bearing (not shown) positioned approximately at the test head""s center of mass. The spherical bearing has an inner race coupled to the central stiffener 212 and an outer race coupled, via the central blade, to the twist bearing 214.
Semiconductor manufacturers and semiconductor testing facilities place a high premium on minimizing the floor space that their ATE systems occupy. As semiconductor devices become more complex, however, the ATE systems used to test them tend to become larger, requiring more floor space. We have recognized that relatively small test heads (e.g., less than 200 kg) can be held effectively with fork arms that do not occupy much additional floor space. For larger test heads, however, fork arms are required to grow substantially in size, to the point where they occupy a significant percentage of the ATE""s overall footprint.
The internally supporting manipulator, like the one shown in FIG. 2, tends to hold heavier test heads with less floor space than an equivalent fork arm manipulator would require. ATE systems now include test heads weighing over 1300 kg. For these larger test heads, internally supporting manipulators are generally the more space efficient alternative.
We have recognized that the centrally supporting manipulator places many constraints on the design of the test head that it supports. For instance, the test head generally must be provided in two portions 210a and 210b, which independently attach to the central stiffener 212. Electrical cables connecting the two portions must be passed either through the central stiffener or around it. Because the central stiffener and associated hardware occupy the center of the test head 210, this area is not available for other purposes, such as cooling and additional electronics.
What would be desirable is a manipulator that is more space efficient for heavy test heads than the fork-arm manipulator but does not impose the design constraints associated with the centrally supporting manipulator.
In accordance with the present invention, a manipulator for supporting a test head includes a body and an interface for supporting the test head from one of its faces, for example, from the rear. The interface includes a first portion fixedly attached to the body and a second portion fixedly attached to the test head at one of its faces. The first and second portions of the interface are rotatably coupled together to allow rotation of the test head about its approximate center of mass. Because the test head is rotatable about its center of mass, it can be made to rotate in response to relatively small applied forces, to satisfy the requirements of compliant docking.